Indeed, silica gel bags are found on almost any new thing ranging from shoes and bags to high end electronics. These bags are also reusable, just use a dry heat source and it will drive the water out of them.
That doesn't work very well, even sitting on a radiator in my house all day I don't get the last 40% out and it is that 40% which works best, the last 40% is very slow at adsorbing moisture, much better to stick them in your microwave oven for 10-20 minutes on simmer (very low), that will get them 100% dry.
If you know the dry weight then you can use the kitchen scales to measure how much water they have inside them, otherwise keep going until they stop getting lighter and then remember that as the dry weight. Put a cup of water in the microwave too, cheap microwave ovens can overheat and kill themselves when run with no water inside, although any microwave oven should be OK on low power.
As I live somewhere which is often damp, I normally keep my cameras inside a sealed plastic box with a large bag of silica gel inside. If you start with a camera that is already full of dry air then there shouldn't really be any need to put silica gel in the waterproof case during diving, there is not a lot of spare room in the Gitup case so drying it out beforehand is easier - put the camera inside the waterproof case inside the plastic box, but leave the waterproof case door open, then close the door before removing it from the plastic box.